.HolyGrail {
    display: flex;
    min-height: 100vh;
    flex-direction: column;
}

header{
    flex:1;
    background-color: red;
}

footer {
    flex: 1;
    background-color: blue;
}

.HolyGrail-body {
    /*display: flex;*/
    flex: 2;
    background-color: gray;
}

/*.HolyGrail-content {*/
    /*flex: 1;*/
/*}*/

/*.HolyGrail-nav, .HolyGrail-ads {*/
    /*!* 两个边栏的宽度设为12em *!*/
    /*flex: 0 0 12em;*/
/*}*/

/*.HolyGrail-nav {*/
    /*!* 导航放到最左边 *!*/
    /*order: -1;*/
/*}*/